This invention relates to a process for converting UF.sub.6 to UO.sub.2.
Uranium for use as fuel for current light water reactors must be isotopically enriched to 2.5-3.0% U.sup.235 from its naturally occurring level of 0.7% U.sup.235. At present this is commercially accomplished through a gaseous diffusion scheme while next generation approaches involve the use of gas centrifuges. The material used in both instances is uranium hexafluoride, UF.sub.6. After enrichment, the UF.sub.6 must be converted to UO.sub.2 which is the material widely used as nuclear reactor fuel.
Presently, there are several methods to effect this conversion. Among these are:
1. The dry process, in which gaseous UF.sub.6 is initially hydrolyzed with dry steam in a fluidized bed to UO.sub.2 F.sub.2 which is subsequently reduced in the same or another fluidized bed to UO.sub.2 employing H.sub.2 as the reducing agent.
2. The ADU process, where UF.sub.6 is hydrolyzed with excess water to form UO.sub.2 F.sub.2 /HF aqueous solutions to which is added aqueous ammonia to effect precipitation of an Ammonium DiUranate which is separated from the liquid phase, dried, and then reduced to UO.sub.2. (Alternatively, the hydrolysis solution may initially contain the ammonia to effect instantaneous precipitation following hydrolysis).
3. The AUC process which is similar to the ADU process but in which ammonium carbonate is employed to precipitate an Ammonium Uranyl Carbonate which is subsequently separated from the reaction solution, dried, calcined, and reduced to UO.sub.2.
The ADU process which is the most widely used conversion method is plagued by such problems as uncontrollable stoichiometry of the resulting precipitates due to minor upsets in operating conditions (e.g., ammonium diuranate can precipitate with NH.sub.4 /U ratios other than 1.0 as required by (NH.sub.4).sub.2 U.sub.2 O.sub.7). This can, and does, affect the physical characteristics of the isolated material. This subsequently effects the sinterability of the resulting UO.sub.2, an extremely critical parameter.
Also large amounts of liquid wastes and by-products are generated which must be handled and disposed of using environmentally acceptable methods. The AUC process also is plagued with problems similar to the ADU process.
The dry fluidized bed process can be hindered by problems arising from the inherent properties of the intermediate solid material, UO.sub.2 F.sub.2 (e.g., the hygroscopic nature of the UO.sub.2 F.sub.2 and its propensity to cake can cause plugging of piping and reactors and can make solids transfer a very difficult operation). Also the UO.sub.2 produced by the dry method has low surface area, spherical particles which are difficult to sinter to the desired density.
The novel process disclosed herein avoids the problems of non-stoichiometry, handling and sinterability. The process involves an intermediate compound derived from (1) the hydrolysis of UF.sub.6 with water, and (2) the precipitation of the resulting UO.sub.2 F.sub.2 from solution with a base. While UO.sub.2 F.sub.2 is formed as an aqueous solution in the ADU process, no effort to utilize this UO.sub.2 F.sub.2 in solution as an isolatable intermediate is made. This is due to its extremely high solubility in excess H.sub.2 O which causes crystallization to be much less than quantitative and results in hard to handle syrups and semisolids very prone to caking if complete evaporation is attempted.